1. Field of the Invention
The present invention is generally related to fluid treatment systems and, more particularly, to flow control in a pressure swing adsorption system.
2. Discussion of the Background
Pressure swing adsorption (PSA) is a commonly used process for the purification of gases. Among other applications, PSA is used to separate oxygen and nitrogen from air, to separate pure helium from natural gas, and to purify hydrogen from ‘reformate’ gas mixtures. The most efficient PSA processes utilize multiple vessels (also referred to as beds) that interact through numerous stages to yield product continuously and at high pressure.
A typical PSA cycle includes a production step in which gas flows into the vessel at high pressure and certain species of the gas are adsorbed while the product species passes through the column. After the production step, pressure equalization steps are used to transfer product gas present in the void space of a high-pressure vessel to other vessels in the system. These pressure equalization steps preserve pressure energy in the system and improve product recovery. The product gas in one vessel may also be used to purge another vessel. Pure product from the vessel providing purge gas sweeps through the vessel being purged at low pressure and, in the process, desorbs and removes the contaminant gases that had been adsorbed during the production step, thus cleaning the vessel. After the vessel has provided purge gas to clean another vessel, it goes through a ‘blowdown’ step in which the pressure in the vessel is rapidly reduced in order to desorb some of the contaminant gases. The vessel is then purged and repressurized with gases from other vessels. The adsorbent bed has then completed its pressure swing cycle and is ready to begin producing gas again. Numerous pressure swing cycles have been devised using two or more adsorbent beds, and all of them use some variants of the steps described above.
In all of the steps of the PSA cycle, it is important to control the flow rate of gases into and out of the vessels and to provide good flow distribution within each vessel. Control of the flow rate in the vessels is required to balance adsorption and diffusion kinetics with production rate and to prevent fluidization of adsorbent particles and pressure shocks within the vessels. In addition, one of the most important flow-related factors in all PSA processes is the purge-to-feed ratio. This ratio is typically defined as the actual volume of purge gas used versus the actual volume of feed gas. The purge-to-feed ratio affects both product purity and product recovery, as higher purge-to-feed ratios indicate that more gas is being used to clean out the bed and therefore product purity is increased at the cost of product recovery.
One way to increase the purge-to-feed ratio and maintain product recovery is to reduce the pressure in the vessel being purged. Reducing pressure decreases the equilibrium surface concentrations of contaminants and provides more volume exchanges in the purged vessel for the same number of moles of purge gas. Therefore, it is advantageous to maintain a low pressure in the purging vessel in order to obtain good product purities with high recoveries. Since the vessel providing purge gas is at high pressure, if no measures are taken to control the flow from this vessel into the vessel being purged, the purge step will have a high initial pressure and therefore reduced effectiveness.
Some PSA systems employ proportional valves, pressure regulating valves, or manually adjustable valves to throttle flow between the vessel providing purge gas and the vessel being purged. Thus, the gas is reduced in pressure before it enters the vessel being purged. Adjustable valves add extra expense to PSA systems and they require extensive tuning to optimize performance. The tuning of adjustable valves is especially difficult and time consuming for low molecular weight product gases, such as hydrogen, since small changes in valve lift lead to large changes in flow rate.
In the absence of the pressure-reducing valves used in some PSA systems, the gas in the vessel providing purge gas rapidly flows into the vessel being purged, usually expanding through one or more fluid shocks and eventually being throttled by the vessel plumbing. This can disadvantageously cause the pressures in the two vessels to equilibrate, thereby causing two separate undesirable phenomena. First, the resulting high pressure in the vessel being purged limits the thermodynamically-feasible extent of desorption of the adsorbed impurities, thus reducing the amount of desorption achieved per mole of purge gas. Second, the rapid decrease in pressure in the vessel providing the purge gas causes a particularly undesirable sudden desorption of adsorbed impurities, which are inevitably carried into the vessel being purged. These impurities then are adsorbed near the critical product outlet end of the vessel being purged and are subject to release during the subsequent production steps, thus significantly reducing product purity.
Therefore, it is undesirable to allow the vessel providing purge gas to rapidly discharge its contents into the vessel being purged. It is likewise undesirable to use the pressure regulating valves described in some PSA systems, as they add complexity to the system. Throttling valves, though less complex than the pressure regulating valves, are also undesirable because they require a fine degree of tuning for each PSA system. This tuning is disadvantageous for PSA systems which are serially-produced, because the process requires time-consuming testing and calibration. These problems are compounded especially for hydrogen-purification PSA systems, as hydrogen has an extremely low viscosity and a very high sonic velocity. These systems require very small orifice sizes to achieve flow throttling relative to other gases. Such small orifices are subject to clogging by adsorbent fines as well as throat erosion and subsequent drift in adjustment. These problems are especially critical in PSA systems processing less than one ton per day of hydrogen, as the nozzle sizes required for throttling become smaller than a single adsorbent particle, and may even be so small as to be unfabricable by ordinary techniques.
Another disadvantage of such throttling devices employing flow through an orifice is that the quantity of flow delivered varies radically from the beginning of the purge step to its end. This is due to the nature of flow through orifices, in that the limiting velocity is proportional to the square of the pressure differential. Thus, the flow rate drops dramatically between the beginning of the time step when the pressure in the vessel providing purge is high and the end of the time step when the pressure is much lower. This rapid change in flow rate can cause both the disadvantageous pressure rise in the vessel being purged and the desorption of impurities in the vessel providing purge. Further, if the desorption kinetics are relatively slow, it can reduce the efficacy of the purge step in removing adsorbed impurity species from the vessel being purged, as much of the purge gas moves through the vessel in the very early part of the purge step.
These challenges are compounded when especially compact PSA systems are desired, due to the complex fluid manifolding required to connect the throttling or pressure regulating valves. Such manifolding disadvantageously increases the manufacturing and assembly time required to produce the PSA system.